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<h3><a name="SECTION001614116000000000000000"></a><a name="ctypes-incomplete-types"></a>
<br>
14.14.1.16 Incomplete Types
</h3>

<p>
<em>Incomplete Types</em> are structures, unions or arrays whose members are
not yet specified. In C, they are specified by forward declarations, which
are defined later:
<div class="verbatim"><pre>
struct cell; /* forward declaration */

struct {
    char *name;
    struct cell *next;
} cell;
</pre></div>

<p>
The straightforward translation into ctypes code would be this, but it
does not work:
<div class="verbatim"><pre>
&gt;&gt;&gt; class cell(Structure):
...     _fields_ = [("name", c_char_p),
...                 ("next", POINTER(cell))]
...
Traceback (most recent call last):
  File "&lt;stdin&gt;", line 1, in ?
  File "&lt;stdin&gt;", line 2, in cell
NameError: name 'cell' is not defined
&gt;&gt;&gt;
</pre></div>

<p>
because the new <code>class cell</code> is not available in the class statement
itself.  In <code>ctypes</code>, we can define the <code>cell</code> class and set the
<tt class="member">_fields_</tt> attribute later, after the class statement:
<div class="verbatim"><pre>
&gt;&gt;&gt; from ctypes import *
&gt;&gt;&gt; class cell(Structure):
...     pass
...
&gt;&gt;&gt; cell._fields_ = [("name", c_char_p),
...                  ("next", POINTER(cell))]
&gt;&gt;&gt;
</pre></div>

<p>
Lets try it. We create two instances of <code>cell</code>, and let them point
to each other, and finally follow the pointer chain a few times:
<div class="verbatim"><pre>
&gt;&gt;&gt; c1 = cell()
&gt;&gt;&gt; c1.name = "foo"
&gt;&gt;&gt; c2 = cell()
&gt;&gt;&gt; c2.name = "bar"
&gt;&gt;&gt; c1.next = pointer(c2)
&gt;&gt;&gt; c2.next = pointer(c1)
&gt;&gt;&gt; p = c1
&gt;&gt;&gt; for i in range(8):
...     print p.name,
...     p = p.next[0]
...
foo bar foo bar foo bar foo bar
&gt;&gt;&gt;
</pre></div>

<p>

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